This invention relates to electrostatic duplication or copying processes. More particularly, this invention provides photocopies with reduced gloss and a method and apparatus for producing such copies.
Electrostatic copying processes generally comprise the steps of electrostatically charging a selectively photo-dischargeable permanent or expendable member in the absence of visible light, exposing the charged member to selectively reflected visible light to create thereon a latent (i.e., invisible) charge pattern comprising electrostatically charged areas on a discharged background, developing the latent charge pattern with visible electroscopic imaging powder (known in the art as "toner") which is electrostatically attracted to the charged areas of the pattern, and permanently fixing the electroscopic powder. In the so called, "transfer process" a permanent photo-dischargeable member is employed, the developed charge pattern (i.e., the electrostatically attracted toner) being transferred to an expendable substrate, e.g. paper, before the toner is fixed. In the "direct process", the photo-dischargeable member comprises an expendable electrostatic copy paper, the toner being permanently affixed to the copy paper without transfer. The present invention provides a method and apparatus for fixing toner powders in both direct and transfer processes.
Methods of permanently affixing toners to photo-dischargeable substrates generally fall into two classes. The first of these methods is the so called heat-setting or heat-fusing method. In this method, a toner comprising a thermoplastic resin is employed. The thermoplastic toner powder is affixed to the substrate by exposing the developed latent image to heat (e.g., 100.degree. C. to about 200.degree. C.) for a time sufficient to soften the toner particles so that they adhere to one another and to the substrate. Heat is usually applied to means of a roller maintained at a pressure of less than about 15 lb/linear inch, 3 kg/linear cm. A second method used to permanently affix toner powders to substrates is pressure fixing. This second technique involves the application of pressures in the range of 25 to 400 pounds per linear inch (5 to 75 kg/linear centimeter) by means of unheated, engaging pressure means, e.g. rollers.
Heat fusing of toner powders, while producing effective results in many cases, is undesirable in that it requires a continuous supply of heat. Continuous heating requires large quantities of electricity, and, in some cases, presents the possibility of overheating the substrate as well as the toner. Additionally, heat fusing or heat fixing of toner powders to substrates generally requires a "warm-up" time period in which the heat-setting members are warmed to the temperature at which toner powders may be heat set. This "warm-up" period usually means a waiting period between the time when the duplicating machine is turned on and when the first copy can be made. Since pressure fixing of toner powders suffers neither of the disadvantages of heat fusing of toners, from the standpoint of energy consumption and convenience it is generally preferable that pressure fixing of toner powders be employed.
Devices for the pressure fixing of toners are known in the art, e.g., U.S. Pat. No. 3,854,975, and U.S. Pat. No. 3,846,151. As is discussed in the U.S. Pat. No. 3,854,975, pressure fixing of toner powders requires a fairly high "load" of toner powder onto the imaging substrate. High loads (i.e., the use of large amounts of toner powder) are required in order to ensure proper fixing of toner powder to the substrate, which usually has substantial variations in surface roughness. Variation in surface roughness of the substrate also requries the use of high fixing pressures to ensure the toner powder is fixed to the substrate in all imaged portions. Extremely smooth pressure rollers have been suggested, e.g. U.S. Pat. No. 3,854,975, as a means to fix toner powders to relatively rough substrates. Smooth pressure rollers and high fixing pressures suffer the disadvantage that they tend to produce imaged substrates with unwanted surface characteristics, i.e., photocopies with shiny or glossy areas.